In the recent past, Signode Corporation, the assignee of the entire interest of the present application, has developed several processes and tools for joining the overlapping end portions of a tensioned thermoplastic loop by friction-fusion techniques, and these methods and apparatuses are typified by those disclosed and claimed in the U.S. patent to Stensaker et al., U.S. Pat. No. 3,442,732; to Vilcins, U.S. Pat. No. 3,442,733; to Ericsson, U.S. Pat. No. 3,442,734; to Stensaker, U.S. Pat. No. 3,442,735; to Kobiella, U.S. Pat. No. 3,442,203; and to Ericsson, U.S. Pat. No. 3,586,572.
Some strapping machines, such as the machine disclosed in the Kobiella U.S. Pat. No. 3,442,203, are of the completely automatic type, i.e., one which automatically feeds a strap around a package from a strap supply source, grips the leading end of the strap, withdraws the standing length of the strap to tension the strap loop, friction-fuses the overlapping portions of the loop, and severs the loop from the standing length of the strap. However, this type of automatic strapping machine has a relatively large, ring-like, rigid chute into which the package is inserted and in which the strap is fed to form a closed loop around the package. With thermoplastic strap, problems have been encountered wherein the strap may buckle or jam in the chute as the strap is fed around the package. This is due to the relatively low column strength of the thermoplastic strap. In addition, a ring-like chute adds considerable bulk to the machine and requires a work space, or operating space, large enough to accommodate the chute and large enough to provide insertion and removal areas for the package.
It would be desirable to provide an automatic strapping machine which could form the strap loop without the need for a large, ring-like chute into which a package must be inserted. Further, such a machine could be made relatively small so that it could be used on a work table or desk for strapping small packages.
Automatic strapping machines which use ring-like strap chutes to form the loop about the package are somewhat inefficient with respect to strapping different size packages. For example, if a strapping machine is intended to strap large packages, say three feet in diameter, then the strap chute must be at least three feet in diameter. If, subsequently, the machine is used to strap much smaller packages, say one foot in diameter, then the strap loop formed around the smaller packages is initially three feet in diameter and the machine must withdraw a substantial amount of trailing strap during the tensioning process to decrease the diameter of the loop and tighten it about the one-foot diameter package. This is obviously inefficient. Therefore, it would be desirable to provide a strapping machine which could form a strap loop of any desired size. Such a machine would advantageously be used in strapping operations where the size of the packages would vary.
Thermoplastic strapping machines currently available also suffer from some drawbacks relating to the formation of the friction-fusion weld. In order to form a friction-fusion weld on the overlapped portion of the strap loop, an anvil or bearing member must be inserted between the package and the strap loop to provide a rigid bearing surface against which the overlapping strap portions are pressed by an oscillating weld member. The anvil member prevents the strap loop from lying flat against the surface of the package at that point and therefore introduces slack into the loop. However, owing to the flexibility of the strap, a tight loop can usually be obtained with large and slightly resilient packages. Unfortunately, though, with very small packages and/or with packages having relatively rigid, incompressible surfaces, the amount of slack introduced into the strap loop by the inserted anvil can be significant and can result in a loose strap loop when the anvil is retracted from between the strap and the package. Accordingly, it is desirable to provide a method and apparatus for welding the strap loop in a manner that does not require the insertion of an anvil member between the strap and the package.